1. Field of the Invention
The present invention is related to an epoxy resin formulation and more specifically to an epoxy resin formulation useful as an insulating material for electrical apparatuses.
2. Description of Background and Related Art
There are several known prior art processes related to filled epoxy for use as electrical insulation materials in which one property of the insulation material is improved. However, heretofore nothing in the known art has successfully provided a filled epoxy resin formulation for electrical insulation materials having the requisite balance of properties such as electrical, mechanical, and thermal properties required for a predetermined operating temperature.
Typical cured epoxy casting insulation material formulations with silica filler have a glass transition of about 70-95° C., tensile strength of about 70-90 MPa, thermal conductivity of generally about less than 1.0 W/mK, volume resistivity of about 1015 to 1016 ohm-cm, including a number of other properties. Solid insulation materials for various end uses need a requisite balance of these properties such as the aforementioned properties. In addition, insulation materials should have processing characteristics including for example the requisite viscosity prior to cure of the epoxy formulation to be a successfully used as an insulation material for example in the field of electrical apparatuses such as power transformers.
The glass transition temperature (Tg) is an especially critical property in the insulation in that as the glass transition is approached (and subsequently passed) mechanical and thermomechanical properties, as measured through a temperature sweep using a Dynamic Mechanical Analyzer or measurement in an environmental chamber at some elevated temperature or temperature ramp, such as storage modulus decrease, tensile strength decrease; electrical insulation properties such as volume resistivity and dielectric strength decrease; and coefficient of thermal linear expansion increases. The changes in the above combination of properties may lead to the premature failure of the insulation material which, in turn, leads to shorting out. For example, Journal of Applied Polymer Science, 1981, 26, 2211 describes a decrease in dielectric strength as Tg is approached in cured epoxy resins. It is also known that a decrease in tensile strength and increase in coefficient of thermal linear expansion of cured epoxy resin products occurs as Tg is approached. Increasing the Tg of an epoxy insulation material is a way to increase the use temperature (including short term higher temperature incursions) of for example a power transformer. However, as Tg is increased in cured epoxy resins, the strength of the material decreases; and thus, brittleness can increase in the insulation material making the insulation material more susceptible to cracking. As a result, cracking contributes to the failure of electrical insulation materials as evidenced by electrical shorting.
It would be useful for electrical insulation applications to develop a filled epoxy formulation having a balance of properties such as Tg, tensile strength; volume resistivity; dielectric breakdown strength and thermal conductivity such that the insulation with the balance of required properties such as electrical, mechanical, and thermal properties can be used at a predetermined continuous operating temperature.